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Asset Impairment Charges and Reversals (Details) $ in Millions, $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2020 CAD ($) | Sep. 30, 2018 CAD ($) | Dec. 31, 2020 CAD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 CAD ($) | Dec. 31, 2018 CAD ($) | Dec. 31, 2012 CAD ($) | Jun. 28, 2018
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Impairment loss | $ 70 |  |  |  |  |  |  |
Write-downs (reversals of write-downs) of property, plant and equipment |  |  | $ 0 |  | $ 18 | $ 23 |  |
Gross carrying amount |  |  |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Write-downs (reversals of write-downs) of property, plant and equipment |  |  | 81 |  | (431) |  |  |
Disposal groups classified as held for sale |  |  |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Impairment loss |  |  |  |  | 15 |  |  |
Sundance Unit 2 |  |  |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Impairment loss |  | $ 38 |  |  |  |  |  |
Lakeswind |  |  |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Impairment loss |  |  |  |  |  | 12 |  |
Lakeswind | Discount rates |  |  |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Significant unobservable input, assets |  |  |  |  |  |  |  | 0.07
Property, plant and equipment | Centralia Thermal Facility |  |  |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Impairment loss |  |  |  |  |  |  | $ 347 |
Provision for decommissioning, restoration and rehabilitation costs |  |  |  |  | 141 |  |  |
Reversal of impairment loss |  |  |  |  | 151 |  |  |
Property, plant and equipment | Lakeswind |  |  |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Impairment loss |  |  |  |  |  | 11 |  |
Property, plant and equipment | Sundance Unit 3 |  |  |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Impairment loss | 70 |  |  |  |  |  |  |
Property, plant and equipment | BC Hydro Facility |  |  |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Impairment loss | $ 2 |  |  |  |  |  |  |
Property, plant and equipment | Centralia mine and Sundance Unit 1 |  |  |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Provision for decommissioning, restoration and rehabilitation costs |  |  | 3 |  |  |  |  |
Land | Gross carrying amount |  |  |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Write-downs (reversals of write-downs) of property, plant and equipment |  |  | 9 |  | $ 0 |  |  |
Land | Gross carrying amount | Centralia Segment |  |  |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Impairment charges |  |  | $ (9) | $ (7) |  |  |  |
Intangible assets | Lakeswind |  |  |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |  |  |
Impairment loss |  |  |  |  |  | $ 1 |  |

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